This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-141549, filed May 21, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an active auto-focus (AF) of a camera, performed by projecting ranging light onto an object to be photographed, and measuring the distance to the object by reflection light therefrom.
In general, as a ranging device used in a camera, a passive AF which utilizes an image signal of the object is known.
In order to perform ranging by the passive AF, a sensor array for obtaining an image signal of an object and a complicated circuit for carrying out an integration process on an output from the sensor array are required. Further, this technique entails a drawback of a low accuracy in the case where the object has a low brightness.
Under these circumstances, the active AF in which light is projected onto an object to be photographed, and measuring the distance to the object by receiving reflection light therefrom has become popular. The active AF has a simple structure and therefore is low cost, as compared to the passive AF. However, since its mechanism is based on the reflection light signal of the reflection light from the object, if the object is located at a far distance, the reflection light signal becomes low, causing deterioration of the accuracy.
A result (signal level) obtained by the active AF is based on the amount of reflection light. Because the amount of reflection light varies depending upon the reflection coefficient of the object, the result of ranging may differ from one case to another as follows. That is, a person dressed in black and a person dressed in white may have different ranging results even if they are standing at the same position.
In particular, as the position of the object is more remote in distance, the amount of reflection light received becomes low. Therefore, the influence of the reflection coefficient described above, and the influence of noise due to other type of light becomes relatively strong, thus making it impossible to perform accurate ranging. As a result, a photograph which was taken in such a situation becomes out of focus.
As a solution to the above-described drawback, Jpn. Pat. Appln. KOKAI Publication No. Hei 5-11173, for example, discloses a technique of ranging which can cover a near distance to a far distance, which is achieved by a position sensitive detector (PSD) for a near distance and a silicon photocell (SPC) for a far distance, used in place of a conventional position detecting means which receives reflection signal light and converts it photo-electrically. In this technique, the position sensitive detector and the silicon photocell are arranged in the same line which is in parallel to the direction of the base length of the taking lens, and the detection device is switched between the PSD and SPC depending upon the position of the object.
In the above-described Jpn. Pat. Appln. KOKAI Publication No. Hei 5-11173, two types of detectors for detecting the reflection light from an object to be photographed, are set side by side, and they are switched depending upon a situation where the position of the object is at a far distance or a near distance, so as to perform the ranging.
These detectors have different output characteristics (inclination), and therefore they operate on different processing methods in terms of the operation processing section where a control signal is generated to the taking lens. Further, at an instantaneous moment of switching the detector from one to another, the signal level input to the operator processing section is temporarily but drastically decreased or increased, and therefore such a switching moment cannot be obtained as a smooth linear variation.
Consequently, in a case where the object is situated at a very position of the distance where the detector is switched over, the signal level input to the operation processing section is greatly switched. In order to avoid this, there must be an addition circuit for carrying out switching before the signal level is input or carrying out the switching within the circuit itself, and thus a complicated process must be carried out within a short period of time.
Under these circumstances, the present invention proposes a ranging device operating on the basis of active AF capable of performing a high-accuracy ranging, which can be achieved with a simple structure and a low cost. In the present invention, the lowering of the accuracy of AF, caused by the decrease in the light amount of the reflection light from the object, is prevented, and the effect of canceling the reflection coefficient dependency, which can be achieved by the triangular ranging technique, is efficiently utilized, so as to obtain the result of the detection in a smooth way from a near distance to a far distance. Thus, the ranging can be performed regardless of the reflection coefficient of the object.
The object of the present invention is to provide a ranging device operating on the basis of active AF capable of performing a high-accuracy ranging, which can be achieved with a simple structure and a low cost, by preventing the lowing of the accuracy of AF, caused by the decrease in the light amount of the reflection light from the object, and efficiently utilizing the effect of canceling the reflection coefficient dependency, which can be achieved by the triangular ranging technique, so as to obtain the result of the detection in a smooth way from a near distance to a far distance, thus making it possible to perform highly accurate ranging regardless of the reflection coefficient of the object.
In order to achieve the above-object of the present invention, there is provided a ranging device comprising: a light projecting portion for projecting ranging light to an object to be photographed; a light receiving portion for receiving reflection light of the ranging light from the object, and for outputting two signals for detecting an incident position thereof; and a superimposing portion for outputting a signal to be superimposed on one of the two output signals when the object is located at a further remote side than at a predetermined distance.
The light receiving portion includes the first position sensitive detector, the second position sensitive detector as the emphasizing means, directing from the far distance side of the position sensitive detector to the far distance side, and switching portion capable of connecting electrically the far distance side of the first position sensitive detector to the far distance side of the second position sensitive detector, and further an object brightness value calculating circuit for obtaining an object distance to the object in accordance with the output from the light receiving portion, and when the object brightness value calculating circuit judges that the object distance is a remote further than a predetermined distance, the switching portion is electrically connected so as to superimpose an output from the second position sensitive detector onto an output from the first position sensitive detector.
Further, the light receiving portion includes the position sensitive detector, the middle electrode as the emphasizing means, provided on the light receiving surface of the position sensitive detector, between the far distance side and the far distance side, and switch portion capable of connecting electrically the far distance side and the middle electrode, and the switch portion is electrically connected in accordance with the incident position of the reflection light received by the light receiving portion, so as to superimpose a signal obtained by the middle electrode onto a signal output from the far distance side.
In the ranging device having the above-described structure, the emphasizing means for emphasizing a signal of a far distance side, is provided, and thus a signal output from the far distance side electrode of the position sensitive detector is superimposed onto a signal obtained from the emphasizing means, thus increasing the level of the signal value of the far distance side electrode, and reduce the error of the ranging operation.
As the emphasizing means, an auxiliary position sensitive detector (the second position sensitive detector) is provided from a predetermined position directing from the near distance side to the far distance side of the original position sensitive detector (the first position sensitive detector) to the far distance side, and a signal obtained by the second position sensitive detector is superimposed on a signal output from the far distance side of the first position sensitive detector to emphasize it.
For the superimposing operation, switch portion made of a switch is provided between the far distance side of the second position sensitive detector and the far distance side of the first position sensitive detector. An output from the first position sensitive detector, or specifically, a signal obtained by the second position sensitive detector by switching the switch portion in accordance with the incident position of the reflection light of the first position sensitive detector, is superimposed onto a signal output from the far distance side of the first position sensitive detector.
As another emphasizing means, a middle electrode is disposed between the end portion of the near distance side and the far distance side on the light receiving surface of the position sensitive detector of the light receiving means, and switch portion made of a switch is provided between the far distance side and the middle electrode. An output from the first position sensitive detector, or specifically, a signal obtained by the middle electrode by switching the switch portion in accordance with the incident position of the reflection light of the first position detection element, is superimposed onto a signal output from the far distance side of the position sensitive detector.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.